For liquid-liquid counterflow extraction, use is made of extraction columns comprising a cylindrical housing in which sieve plates are fixed rigidly and in an axially distributed arrangement. The lighter phase is supplied from below while the heavier phase is supplied from above. The heavy phase is discharged at the bottom of the column while the light phase is discharged at the top of the column.
The extraction column for carrying out a liquid-liquid counterflow extraction must be adapted by means of special features to the specific requirements of the extraction process in order to accelerate the transfer of material between the phases and to increase operational reliability and availability as well as the efficiency.
It is known for sieve plates to be arranged in a rigidly fixed and axially distributed arrangement in the cylindrical housing of the extraction column. The sieve plates are intended to provide an adequate degree of quality of extraction by virtue of the fact that they cause droplets of liquid of the one phase to be formed on passing through the holes in the sieve plates. The droplet formation increases the size of the surface area of the phase. The exchange of material is advantageously affected.
In order to further improve the level of efficiency of the sieve-plate column, it is known to impart a pulsation to the liquid content. Pulsed sieve-plate columns can be of shorter height, with the same level of efficiency, than non-pulsed sieve-plate columns. However, it has been observed that the level of efficiency that was theoretically expected in the extraction operation was not achieved. As a good transfer of material depends on good mixing of the two phases, the unsatisfactory effect was hitherto attributed to re-mixing of the two phases with each other, which can occur because of a return movement of the column of liquid. That assumption was not an adequate explanation for the unsatisfactory level of efficiency as was noted in a presentation at the International Meeting Fuel Reprocessing and Waste Management, Aug. 26th to 29th, 1984, Jackson, Wyo., page 5.
In addition, it has now been found that phase channels which are formed in a flow path configuration pass vertically through the sieve-plate column. This phenomenon becomes more pronounced with increasing column diameter so that a greater length of column is required for extraction.
It was realized that the above-mentioned channel flow phenomenon must be brought to an end in order to achieve improved material exchange. In this way, consideration is taken of the situation that the driving force and speed in a material exchange in extraction columns is the concentration gradient between the receiving phase and the delivering phase. This concentration gradient should be as high as possible. In order to achieve this gradient, it is important to constantly destroy the boundary layers of the phases to bring non-loaded or less loaded phase particles to the droplets of the other phase. This can be achieved by the phases as they pass through the column in that the phases are repeatedly diverted out of the flow paths and mixed radially as far as possible. Radial mixing results in destruction of the channel flow and thereby provides a better degree of material exchange.
German published and examined patent application DE-AS No. 17 69 005 describes a column arrangement having rotatable mixing arrangements disposed between sieve plates for cross-mixing the phases. The disadvantage of these rotatable mixing arrangements lies in maintenance of the bearings and the complicated configuration thereof. When the sieve-plate column is used in nuclear reprocessing, movable components in the sieve-plate column are undesirable.
A further sieve-plate column is disclosed in the technical journal Chemical Engineering Prog. Symposium Series, 13, Vol. 50, (1954), pages 14 to 17. This sieve-plate column does not require any rotatable or movable components in the mixing arrangement. This mixing arrangement comprises a distributor plate which is disposed between the sieve plates. Angle plates project out of the plane of the distributor plate at an inclined position and at an angle of about 30.degree.. The angle plates are bent out of the distributor plate so that apertures are present beneath the angle plates and are covered by the latter. This configuration is intended to impart a swirl to the liquid of each pulse. This distributor plate provides for a good cross-mixing effect and thus an improved material exchange between the phases. The use of the distributor plate results in an improved distribution of the concentration of material over the corresponding ones of the cross-sections of the sieve-plate column, which also has an advantageous effect on the material exchange between the phases. However, complete or adequate prevention of cross-mixing non-homogeneity cannot be achieved with this distributor plate. In this connection, reference may be made to a presentation at the International Meeting Fuel Reprocessing and Waste Management, Aug. 26th to 29th, 1984, Jackson, Wyo., page 8.
In order to produce the swirl effect, the free area of the distributor plate must be less than the free area of the sieve plate. A serious disadvantage is seen in the distributor plate because it considerably reduces the free through-flow cross-section of the sieve-plate column. The sieve nozzle-type plates which are used in the nuclear reprocessing art usually have a through-flow cross-section of about 23% (Presentation at the Internation Meeting Fuel Reprocessing and Waste Management, Aug. 26th to 29th, 1984, Jackson, Wyo., page 5). The distributor plate only has a through-flow cross-section of about 14% (Presentation at the International Meeting Fuel Reprocessing and Waste Management, Aug. 26th to 29th, 1984, Jackson, Wyo., page 8). If the through-flow cross-section in the sieve-plate column becomes less, the result is an undesirable drop in through-put.
Another disadvantage is considered to lie in the constricted operating range of a sieve-plate column which includes the distributor plate.
The smaller free surface area of the distributor plate can also result in a backup of liquid on the plate, which under some circumstances can result in deposits on the distributor plate.